常用气体地热温度计的应用及效果评价

基于冰岛部分地热田的实际资料,选择有代表性的、不同温度的地热田,应用CO₂、H₂S、H₂、CO₂/H₂和H₂S/H₂气体地热温度计计算热储温度,深入探讨了影响温度计准确性的各种因素,提出CO₂和H₂S温度计具有良好的实用价值。在热储温度T<200℃时,CO₂温度计的预测温度略低;在200℃<T<300℃范围时,H₂S温度计标定不够准确。并对现有的其它气体地热温度计进行了简要评价。     

Geothermal gas equilibria

Comparison of theoretical and analytical equilibrium constants based on the reactions CH₄ + 2H₂O = CO₂ + 4H₂, 2NH₃ = N₂ + 3H₂ and iron(II)-aluminium-silicate + 2H₂S = FeS₂ + H₂ + aluminium-silicate, shows that the composition of fluids discharged from geothermal areas in New Zealand (Wairakei, Kawerau, Broadlands) reflects close to complete attainment of chemical equilibrium within the system H₂O, CO₂, H₂S, NH₃, H₂, N₂ and CH₄. Under conditions prevailing in explored geothermal systems in New Zealand, the minerals graphite (elemental carbon), anhydrite, pyrrhotite, magnetite do not appear to take part in the overall equilibrium system. The three physical parameters required to evaluate geothermal gas reactions are temperature, pressure and vapor-liquid ratios within the gas equilibration zone.     

Seawater sulfate reduction and sulfur isotope fractionation in basaltic systems: Interaction of seawater with fayalite and magnetite at 200–350°C

Sulfate reduction during seawater reaction with fayalite and with magnetite was rapid at 350°C, producing equilibrium assemblages of talc-pyrite-hematite-magnetite at low water/rock ratios and talc-pyrite-hematite-anhydrite at higher water/rock ratios. At 250°C, seawater reacting with fayalite produced detectable amounts of dissolved H₂S, but extent of reaction of solid phases was minor after 150 days. At 200°C, dissolved H₂S was not detected, even after 219 days, but mass balance calculations suggest a small amount of pyrite may have formed. Reaction stoichiometry indicates that sulfate reduction requires large amounts of H⁺, which, in subseafloor hydrothermal systems is provided by Mg metasomatism. Seawater contains sufficient Mg to supply all the H⁺ necessary for quantitative reduction of seawater sulfate.Systematics of sulfur isotopes in the 250 and 350°C experiments indicate that isotopic equilibrium is reached, and can be modeled as a Rayleigh distillation process. Isotopic composition of hydrothermally produced H₂S in natural systems is strongly dependent upon the seawater/basalt ratio in the geothermal system, which controls the relative sulfide contributions from the two important sulfur sources, seawater sulfate and sulfide phases in basalt. Anhydrite precipitation during geothermal heating severely limits sulfate ingress into high temperature interaction zones. Quantitative sulfate reduction can thus be accomplished without producing strongly oxidized rocks and resultant sulfide sulfur isotope values represent a mixture of seawater and basaltic sulfur.     

Hydrochemistry and hydrogen sulfide generating processes in the Malm aquifer,Bavarian Molasse Basin,Germany

Knowledge about the hydrochemical conditions of deep groundwater is crucial for the design and operation of geothermal facilities. In this study, the hydrochemical heterogeneity of the groundwaters in the Malm aquifer, Germany, is assessed, and reasons for the extraordinarily high H₂S concentrations in the central part of the Bavarian Molasse Basin are proposed. Samples were taken at 16 sites, for a total of 37 individual wells, to analyze cations, anions, gas loading and composition. The hydrochemical characteristics of the Malm groundwater in the center of the Molasse Basin are rather heterogeneous. Although the groundwater in the central basin is dominated by meteoric waters, there is a significant infiltration of saline water from higher strata. Care has to be taken in the interpretation of data from geothermal sites, as effects of chemical stimulation of the boreholes may not be fully removed before the final analyses. The distribution of H₂S in the gas phase is correlated to the gas loading of the water which increases in the central basin. Temperatures, isotopic data and the sulfur mass balance indicate that H₂S in the central basin is related to thermochemical sulfate reduction (south of Munich) and bacterial sulfate reduction (north of Munich).     

New gas geothermometers for geothermal exploration—calibration and application

Calibration of five gas geothermometers is presented, three of which used CO₂, H₂S and H₂ concentrations in fumarole steam, respectively. The remaining two use $\text{CO}{}_2\text{H}{}_2$ and $\text{H}{}_2\text{S}\text{H}{}_2$ ratios. The calibration is based on the relation between gas content of drillhole discharges and measured aquifer temperatures. After establishing the gas content in the aquifer, gas concentrations were calculated in steam formed by adiabatic boiling of this water to atmospheric pressure to obtain the gas geothermometry functions. It is shown that the concentrations of CO₂, H₂S and H₂ in geothermal reservoir waters are fixed through equilibria with mineral buffers. At temperatures above 230°C epidote + prehnite + calcite + quartz are considered to buffer CO₂. Two buffers are involved for H₂S and H₂ and two functions are, therefore, presented for the geothermometers involving these gases. For waters containing less than about 500 ppm chloride and in the range 230–300°C pyrite + pyrrholite + epidote + prehnite seem to be involved, but pyrite + epidote + prehnite + magnetite or chlorite for waters above 300°C and waters in the range 230–300°C, if containing over about 500 ppm.The gas geothermometers are useful for predicting subsurface temperatures in high-temperature geothermal systems. They are applicable to systems in basaltic to acidic rocks and in sediments with similar composition, but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones. The geothermometry results may be used to obtain information on steam condensation in upflow zones, or phase separation at elevated pressures.Measured aquifer temperatures in drillholes and gas geothermometry temperatures, based on data from nearby fumaroles, compare well in the five fields in Iceland considered specifically for the present study as well as in several fields in other countries for which data were inspected. The results of the gas geothermometers also compare well with the results of solute geothermometers and mixing models in three undrilled Icelandic fields.     

Reconstruction of Physicochemical Environment of Hydrothermal Mineralization at Malanjkhand Copper Deposit,Central India: Constraints from Sulfur Isotope Ratios in Pyrite,Molybdenite and Chalcopyrite

δ³⁴S values of pyrite, molybdenite and chalcopyrite were determined from the Malanjkhand copper deposit. These minerals constitute the primary sulfide phases that were deposited after the initial magnetite deposition in the main orebody and host granitoid. Pyrite exhibits a depleted range of values (?2.63 to ?0.56‰), chalcopyrite, a very narrow range of values around zero (?0.039 to 0.201‰) and molybdenite furnishes a range of enriched values (0.68 to 1.98‰). On back calculation of the δ³⁴S values of H₂S in the fluid from which the minerals were likely to have precipitated, using standard expressions for equilibrium fractionation at the temperature range obtained from fluid inclusion and mineral fluid equilibria, it is observed that H₂S in the fluid at pyrite deposition was depleted and gradually became enriched towards molybdenite and chalcopyrite deposition. This trend is best explained as being due to inorganic reduction of SO₄^2? in the fluid and is very much in agreement with the paragenetic sequence indicating increasing activity of H₂S in the fluid. The very restricted range in the δ³⁴S values of sulfide minerals in the fluid does indicate a single, possibly magmatic, source of sulfur that also agrees well with the earlier deduced model of genesis of the deposit as an ancient geothermal system associated with granitic magmatism.     

河北汤泉地热流体水文地球化学特征及其成因

提要：汤泉位于河北省遵化市西北部,为山前丘陵地貌,地热资源丰富。本文通过对该地区地热流体研究发现：Na⁺、Ca²⁺、K⁺、Mg²⁺与SO^2-₄、HCO^-₃、Cl^-、NO^-₃是该地区地下热水的主要成分,水化学类型主要为SO^2-₄-Na⁺型,属于未污染的天然弱碱性水;流体中F^-含量平均为9.36 mg/l,远高于国家地下水质量标准ⅴ级;可溶性SiO₂的含量可作为地热温标;地热流体总矿化度平均为782.33 mg/l,属于淡水;为中等腐蚀型水,不结碳酸钙垢,无CaSO₄?2H₂O垢和SiO₂垢生成的可能;地热流体属于含岩盐地层溶滤的陆相沉积水;根据氢氧稳定同位素可知,河北汤泉地热流体主要来源于大气降水。     

Hydrogen sulfide and organic carbon at the sediment–water interface in coastal brackish Lake Nakaumi, SW Japan

The relationship among H₂S, total organic carbon (TOC), total sulfur (TS) and total nitrogen contents of surface sediments (0–1 cm) was examined to quantify the relationship between H₂S concentrations and TOC content at the sediment water interface in a coastal brackish lake, Nakaumi, southwest Japan. In this lake, bottom water becomes anoxic during summer due to a strong halocline. Lake water has ample dissolved SO₄ ^2? and the surface sediments are rich in planktic organic matter (C/N 7–9), which is highly reactive in terms of sulfate reduction. In this setting the amount of TOC should be a critical factor regulating the activity of sulfate reduction and H₂S production. In portions of the lake where sediment TOC content is less than 3.5 %, H₂S was very low or absent in both bottom and pore waters. However, in areas with TOC >3.5 %, H₂S was correlated with TOC content (pore water H₂S (ppm) = 13.9 × TOC (%) ? 52.1, correlation coefficient: 0.72). H₂S was also present in areas with sediment TS above 1.2 % (present as iron sulfide), which suggests that iron sulfide formation is tied to the amount of TOC. Based on this relationship, H₂S production has progressively increased after the initiation of land reclamation projects in Lake Nakaumi, as the area of sapropel sediments has significantly increased. This TOC–H₂S relationship at sediment–water interface might be used to infer H₂S production in brackish–lagoonal systems similar to Lake Nakaumi, with readily available SO₄ ^2? and reactive organic matter.     

Seasonal and topographic variations in porewaters of a southeastern USA salt marsh as revealed by voltammetric profiling†

We report electrochemical profiles from unvegetated surficial sediments of a Georgia salt marsh. In creek bank sediments, the absence of ΣH₂S or FeS_aq and the presence of Fe(III)–organic complexes suggest that Mn and Fe reduction dominates over at least the top ca. 5 cm of the sediment column, consistent with other recent results. In unvegetated flats, accumulation of ΣH₂S indicates that SO₄ ^2- reduction dominates over the same depth. A summer release of dissolved organic species from the dominant tall form Spartina alterniflora, together with elevated temperatures, appears to result in increased SO₄ ^2- reduction intensity and hence high summer concentrations of ΣH₂S in flat sediments. However, increased bioturbation and/or bioirrigation seem to prevent this from happening in bank sediments. Studies of biogeochemical processes in salt marshes need to take such spatial and temporal variations into account if we are to develop a good understanding of these highly productive ecosystems. Furthermore, multidimensional analyses are necessary to obtain adequate quantitative pictures of such heterogeneous sediments.     

Application of in-situ and on-line measurements to environmental studies in Lake Müritz,northern Germany

A new system, MINIBAT, for in-situ measurements and continuous collection of water samples was used in Lake Müritz (Mecklenburg-Vorpommern) to make detailed and simultaneous measurements of temperature, conductivity, pressure, particle density, H₂S, O₂, pH, nutrients and heavy metals. The underwater measuring unit consists of a multisensor unit, a micro-processor and an underwater pump (pumping rate up to 0.8 l/min) carried in a small, towed frame. The onboard unit (IBM compatible computer) is able to record sensor and location data transmitted from a GPS system and echosounder. Onboard, the water samples are split for introduction into a H₂S flow-through sensor and for aliquots which are filtered through standard membrane filters and preserved either for trace metal determination or nutrient analysis. All data are stored in a data bank which can be used as a base for mapping the different parameters within the water body. This compact and portable system represents a breakthrough for hydrochemical investigations of lakes and rivers and was used to map the hydrochemical parameters and anthropogenic influences in Lake Müritz. In-situ measurements made during the summer stagnation showed that a 10 m-thick layer enriched in H₂S built up in the bottom water. This anoxic hypolimnion was characterized by a steadily increasing H₂S content approaching the sediment surface and with an associated increase in the contents of the nutrients NH₄⁺ and PO₄³⁻ and a decrease in the heavy metal contents.     

Balance of S in a constructed wetland built to treat acid mine drainage, Idaho Springs, Colorado, U.S.A.

The wetland constructed at the Big Five Tunnel in Idaho Springs, Colorado was designed to remove, passively, heavy metals from acid mine drainage. In optimizing the design of such a wetland, an improved understanding of the chemical processes operating there was required, particularly SO₄²⁻ reduction and sulfide precipitation. For this purpose, field and laboratory data were collected to study the balance of S in the system. Field data collected included water analyses of the mine drainage and wetland effluents and measurements of H₂S gas emissions from the wetland. The concentration of sulfide in the wetland effluent ranged from 10⁻⁴ to 10⁻³ mol/l. The average rates of H₂S emission from the surface of the substrate were 150 nmol/cm²/d in the summer and 0.17 and 0.35 nmol/cm²/d in the winter. This maximum estimated loss of sulfide was not significant in reducing the amount of sulfide available for precipitation with metals. Sequential extraction experiments for S on wetland substrates showed that acid volatile sulfides (AVS) increased with time in the wetland substrate. A serum bottle experiment was conducted to study the S balance in the Big Five wetland by quantitatively measuring the amount of S in different phases as microbial SO₄²⁻ reduction progressed. The increase in AVS reasonably balanced the decrease in SO₄²⁻ concentration in the experiment, suggesting that the decrease in SO₄²⁻ concentration represented the amount of SO₄²⁻ reduced and that nearly all of the sulfide produced was precipitated as AVS. Sulfide precipitation was determined to be the primary metal removal process in the wetland system and amorphous FeS is the primary iron sulfide formed in the substrate.     

Sulphur solubility in andesitic to basaltic melts: implications for Hekla volcano

The solubility of sulphur in sulphide-saturated, H₂O-bearing basaltic–andesitic and basaltic melts from Hekla volcano (Iceland) has been determined experimentally at 1,050°C, 300 and 200 MPa, and redox conditions with oxygen fugacity (logfO₂) between QFM−1.2 and QFM+1.1 (QFM is a quartz–fayalite–magnetite oxygen buffer) in the systems containing various amounts of S and H₂O. The S content of the H₂O-rich glasses saturated with pyrrhotite decreases from 2,500 ppm in basalt to 1,500 ppm in basaltic andesite at the investigated conditions. Furthermore, the reduction of water content in the melt at pyrrhotite saturation and fixed T, P and redox conditions leads to a decrease in S concentration from 2,500 to 1,400 ppm for basaltic experiments (for H₂O decrease from 7.8 to 1.4 wt%) and from 1,500 to 900 ppm (for H₂O decrease from 6.7 to 1.7 wt%) for basaltic andesitic experiments. Our experimental data, combined with silicate melt inclusion investigations and the available models on sulphide saturation in mafic magmas, indicate that the parental basaltic melts of Hekla were not saturated with respect to sulphide. During magmatic differentiation, the S content in the residual melts increased and might have reached sulphide saturation with 2,500 ppm dissolved S. With further magma crystallization, the S concentration in the melt was controlled by the sulphide saturation of the magma, decreasing from ~2,500 to 900 ppm S.     

The roles of anoxia, H2S, and storm events in fish kills of dead-end canals of Delaware inland bays

In 2001, the development of seasonal anoxia was studied in two waterways located at the head of Delaware’s northern inland bay, Rehoboth Bay. Bald Eagle Creek is a northern tributary of the bay, which has tidal exchange with Torquay Canal (a dead-end canal) via a short channel with a 1.4 m sill. Mean low water depth in Torquay Canal is about 2 m, but dredging produced over a dozen depressions with a total water depth of 5.5 m. During the summer of 2000, four major fish kills were reported in Torquay Canal and Bald Eagle Creek with more than 2.5 million juvenile menhaden (Brevoortia tyrannus) killed. Low O₂ concentration was assumed to be the problem but production of toxic H₂S is more likely. From late spring 2001, we conducted in situ determination of temperature, salinity, pH, dissolved O₂, and H₂S in Torquay Canal and Bald Eagle Creek. During spring, water column stratification began in the depressions with warmer and less salty water observed in the upper layer, and cooler, saltier water below 2 m. O₂ was at saturation levels in the surface waters but was not detectable below 2 m by the end of May. The depressions were anoxic with H₂S accumulating to mM concentrations in June. A storm event prior to July 12 mixed these two layers with a subsequent loss of H₂S. The H₂S levels again increased in the deep water due to stratification and reached another maximum in late August. Another storm event occurred at this time resulting in no detectable O₂ and up to 400 μM H₂S in surface waters. H₂S appears to be the primary reason for fish kills in these tributaries. Aerators installed in Torquay Canal on June 21 had no significant effect on abating stratification and anoxic conditions beyond their immediate area.     

Dissolved Fe2+ and ∑H2S Behavior in Sediments Seasonally Overlain by Hypoxic-to-anoxic Waters as Determined by CSV Microelectrodes

Variability of dissolved Fe²⁺ and ∑H₂S concentrations in porewaters were studied, using cathodic stripping voltammetry (CSV) microelectrodes, in sediments overlain by hypoxic waters in the summer from the southeastern region of Corpus Christi Bay, Texas (CCB) and the Mississippi River Bight (MRB), Louisiana. These measurements were complimented by sediment microcosm studies where oxygen concentrations in the overlying water were manipulated. Sulfate reduction rates, benthic oxygen demand, total reduced sulfide, porewater sulfate, and total organic carbon were also determined. Fe²⁺ and ∑H₂S were the major dissolved redox-reactive dissolved species in these sediments. During hypoxic conditions, an upward migration of porewater Fe²⁺ and ∑H₂S occurred, with Fe²⁺ reaching much higher maximum concentrations than ∑H₂S. Statistically significant (p < 0.05) differences in both Fe²⁺ and ∑H₂S occurred between sediments at the CCB and MRB study sites. Although both sites were Fe-dominated, reactive and dissolved iron were higher while ∑H₂S was lower at the MRB site. However, there were no statistically significant (p > 0.05) difference in regard to ∑H₂S between microcosm and field monitoring studies. Results indicated that, for Fe²⁺ and ∑H₂S, relatively large and rapid changes occurred in both the concentrations and distributions of these important porewater constituents in response to relatively short-term changes in overlying water oxygen content. Model calculations indicated that conditions in the sediments can be responsible for the induction of hypoxic conditions in the formation of hypoxic overlying waters in <6 days at CCB and ~20 days at MRB.     

Magnetic susceptibility of volcanic rocks in geothermal areas: application potential in geothermal exploration studies for identification of rocks and zones of hydrothermal alteration

Magnetic susceptibility and petrographic studies of drilled rock cuttings from two geothermal wells (Az-26 and Az-49) of the important electricity-generating geothermal system, Los Azufres, Mexico, were carried out to determine the relation between the magnetic susceptibility of rocks, the concentration of magnetic minerals and hydrothermal alteration. For this purpose, low-frequency magnetic susceptibility (χ _lf) was measured and compared its distribution trends with those of magnetic and Fe–Mg silicate minerals, and with the extent of hydrothermal alteration in rocks of the two geothermal wells. The study indicates a decrease in χ _lf values with depth in the two geothermal wells corresponding with: (1) an increase in the reservoir temperature and hydrothermal alteration; and (2) a decrease in the concentrations of Fe–Mg silicates and opaque minerals. The data suggest that ferromagnesian minerals and opaque minerals like ilmenite are the main contributors to the χ _lf of rocks. The decrease in χ _lf, ilmenite, and Fe–Mg mineral contents with an increase in the hydrothermal alteration degree, pyrite and haematite contents suggests the hydrothermal alteration of ilmenite and Fe–Mg minerals (characteristic of high χ _lf values) to pyrite, haematite and other opaque minerals (with low χ _lf values). The interaction of hydrothermal fluids with rocks results in the hydrothermal alteration of primary minerals. In a geothermal area, an anomaly of low magnetic susceptibility values of rocks in a homogenous litho unit characterized by high magnetic susceptibility may suggest hydrothermal alteration. Magnetic susceptibility can be a useful parameter, during the initial stages of geothermal exploration, in identifying hydrothermally altered rocks and zones of hydrothermal alteration both at the surface and from drilled wells in geothermal systems.     

Water transportation from the subducting slab into the mantle transition zone

Using a recently developed petrogenetic grid for MORB + H₂O, we propose a new model for the transportation of water from the subducting slab into the mantle transition zone. Depending on the geothermal gradient, two contrasting water-transportation mechanisms operate at depth in a subduction zone. If the geothermal gradient is low, lawsonite carries H₂O into mantle depths of 300 km; with further subduction down to the mantle transition depth (approximately 400 km) lawsonite is no longer stable and thereafter H₂O is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection. At this depth, hydrous β-phase olivine is stable and plays a role as a huge water reservoir. In contrast, if the geothermal gradient is high, the subducted slab may melt at 700–900 °C at depths shallower than 80 km to form felsic melt, into which water is dissolved. In this case, H₂O cannot be transported into the mantle below 80 km. Between these two end-member mechanisms, two intermediate types are present. In the high-pressure intermediate type, the hydrous phase A plays an important role to carry water into the mantle transition zone. Water liberated by the lawsonite-consuming continuous reaction moves upward to form hydrous phase A in the hanging wall, which transports water into deeper mantle. This is due to a unique character of the reaction, because Phase A can become stable through the hydration reaction of olivine. In the case of low-pressure intermediate type, the presence of a dry mantle wedge below 100 km acts as a barrier to prevent H₂O from entering into deeper mantle.     

西藏羊八井地热田热水的化学组成

羊八井地热田深、浅层热水都是Cl-Na类型,具有相同的B/Cl比值,说明深层热水在上升通道中与冷水相混合形成了浅层热水。浅层流体自西北向东南流动,温度逐渐降低。浅层热储内普遍存在着水岩交换反应,对热水的化学组成有一定的影响。石英和玉髓地热温度计分别适用于计算深、浅层的热储温度。纳木错(湖)不是羊八井地热田的补给区。深层热水在井筒内绝热汽化时不会出现SiO₂结垢,CaCO₃是否会在井筒壁沉淀需要放喷较长时间来检验。文中还阐述了对热水的化学组分进行监测的必要性。     

Chemical and isotopic evidence for secondary alteration of natural gases in the Hetianhe Field, Bachu Uplift of the Tarim Basin

H₂S and CO₂ are found in elevated concentrations in the reservoirs near the Carboniferous–Ordovician unconformity in the Hetianhe Field of the Tarim Basin, NW China. Chemical and isotopic analyses have been performed on produced gases, formation waters and reservoir rocks to determine the origin of CO₂ and H₂S and to explain the heterogeneous distribution of isotopic and geochemical characteristics of petroleum fluids. It is unlikely that H₂S and CO₂ had a mantle component since associated helium has an isotope ratio totally uncharacteristic of this source. Instead, H₂S and CO₂ are probably the result of sulphate reduction of the light hydrocarbon gases (LHG). Increasing H₂S concentrations and CO₂/(CO₂+ΣC_1–4) values to the west of the Hetianhe Field occur commensurately with increasingly heavy hydrocarbon gas δ¹³C values. However, thermochemical sulphate reduction is unlikely because the temperatures of the reservoirs are too low, no H₂S or rare pyrite was detected in deeper reservoirs (where more TSR should have occurred) and inferred δ³⁴S values of H₂S (from late-stage pyrite in the Carboniferous and Ordovician reservoirs) are as low as −24.9‰. Such low δ³⁴S values discount the decomposition of organic matter as a major source of H₂S and CO₂. Bacterial sulphate reduction of the light hydrocarbon gases in the reservoir, possibly coupled indirectly with the consumption of organic acids and anions is most likely. The result is the preferential oxidation of ¹²C-rich alkanes (due to the kinetic isotope effect) and decreasing concentration of organic acids and anions. Modern formation water stable isotope data reveal that it is possible that sulphate-reducing bacteria were introduced into the reservoir by an influx of meteoric water from the west by way of an inversion-related unconformity. This may account for the apparently stronger influence of bacterial sulphate reduction to the west of the Hetianhe Field, and the consequent greatest decrease of the δ¹³C-CO₂ values and the greatest increase in δ¹³C values of the alkane gases.     

Sulfur dioxide in geothermal waters and gases

Methods were developed for stabilizing SO₂ in water and gas samples. The pararosaniline colorimetric method, and a gas Chromatographic method using a flame photometric detector specific for sulfur gases were used to assay SO₂. Assays were also performed for sulfide, elemental sulfur and sulfate.A large number of acidic, neutral, and alkaline springs in Yellowstone National Park were sampled: SO₂ was found in small amounts in most of them. The highest concentration detected in water was 0.5–0.6 μg/ml (expressed as sulfur). Sulfur dioxide was never detected in gases emanating from hot springs, or in fumaroles, although H₂S was readily detected. Because of the high solubility of SO₂ in water, and its low pK, it is unlikely that environmentally significant quantities are volatilized from geothermal systems of the low-temperature type characteristic of Yellowstone Park. Laboratory studies suggest that in acid waters, ferric iron is the primary oxidant, as H₂S is not oxidized by O₂ at low pH. At neutral or alkaline pH, O₂ is the likely oxidant, because sulfide is oxidized by O₂ at these pH values, and neutral and alkaline hot springs are always low in iron. Although bacteria capable of oxidizing sulfide and elemental sulfur are present in most of the springs sampled, it is concluded that the oxidation of reduced sulfur compounds to sulfur dioxide is primarily a chemical process, because of the rapidity with which it occurs and the lack of any evidence that bacteria produce sulfur dioxide.     

Geothermal energy research in Kenya: a review

Geothermal energy for electricity generation is likely to become increasingly important in Kenya in the future. There are numerous centres of thermal activity in Kenya, particularly within the Rift Valley, although aridity and, consequently, availability of water may be a constraint to the development of large scale natural hydrothermal systems. Geothermal resources in the islands of Lake Turkana and those close to other rift lakes deserve further investigation as they do not suffer from the constraints of a shortage of water. The experience gained so far at Oikaria shows that environmental problems can be adequately addressed, though constant monitoring is necessary. H₂S emissions preclude the setting up of permanent residences within about 5 km of the geothermal power stations. Trace elements and radiation from geothermal fluids need to be monitored with respect to their impacts on plants and animals. The impact on the local hydrogeology also requires close observation. Multistage uses of geothermal fluids will greatly increase the benefits derived from this resource.